Bulletin 7 - April 1979: Life of the Desert
Life of the Desertby Bish Brown
The deserts of the world are vase, covering almost one-fifth of the total land surface. The Arabian Peninsula has a total are of about a million square miles, the greater part of which is are steppe and desert terrain, part of the great Palaearctic Desert tract stretching from the Sahara to Sind in West Pakistan. About one-fifth is sandy desert like that we see on the way to Al Ain, in which the wind has built up great ridges Rub al Khali or Empty Quarter. Other vast tracts are composed of undulating, featureless wastes with stony or dusty surfaces and little permanent vegetation, except in hollows. They are areas of extreme temperatures and little or no water. Rainfall is erratic and is usually less than 25cm (10 inches) per year. The sun beats down from a clear blue sky with no clouds to filter and lessen the heat. There is little or no vegetation for shade. With clear skies at night, the heat absorbed by sand and rocks during the day can radiate back to the sky without clouds to restrict it. The temperature falls quickly, giving extremely cold nights and surface condensation.
For mammals and other creatures these conditions create problems. A few species have adapted to these conditions. Animals and plants have adapted in a great variety of ways, some more efficiently than others. Because vegetation is extremely sparse, this causes a food problem and makes concealment from predators difficult. Conversely, of course, predatory creatures like wolves, foxes and birds of prey have greater difficulty approaching and capturing their prey undetected. Because of this lack of cover, nearly all desert mammals are diurnal or nocturnal, only leaving the security of their burrows or lairs during the low light or dark hours. They have adapted a light desert coloration, which acts as camouflage should they be caught out in daylight. Some of the jerboas have evolved a different form of predator avoidance. By progressive elongation of the hind foot and the elevation of the stance onto the tips of the three toes, a method of moving by rapid irregular jumps has been evolved. This makes the jerboa extremely difficult to capture and allows it to wander further afield. Two species of jerboa are found in the Arabian deserts: the five-toes jerboa which lives on the northern stony-steppe deserts (their outer toes are still present on the foot, but are functionless) and the three-toed jerboa. Tufts of long, rigid hair on the three remaining toes enables them to obtain a sure footing on soft, loose sand. The sandcat has long wavy hairs covered the foot pads and the desert hare shows the same tendency.
The scarcity of vegetation means little food for plant eating animals and this leads to low population density and wide dispersal of individuals. Predators are even more widely dispersed under these conditions. Locating one of their own species becomes difficult and almost without exception desert animals have very large ears. The long chamber of the middle ear is enlarged to sharpen the frequency range. The ear enlargement also probably facilitates greater heat loss from the body, without loss of moisture.
Some animals have evolved visual signals to wan of approaching danger. The jerboa has a prominent black and white tuft on the tail tip which is very obvious as it hops around in moonlight or the fast gathering gloom of the desert dusk. The Arabian Jird, which is active by day, holds its black tipped tail erect as it flees to its burrow. The desert hare has a white tail which it displays prominently as it weaves its way through the clumps of vegetation when it is disturbed. Two or three hares were recently seen in the desert area not far from Abu Dhabi island.
Other important problems which desert mammals must solve are imposed upon them by the climate. The regulation of temperature in small animals is the least difficult of these, because fortunately the intense heat of the desert sun does not penetrate far beneath the surface of the earth. Being close to the ground, animals like the jerboa would quickly absorb heat and die. They retreat to burrows during the daytime and seal the entrance to create a micro-climate. Foxes and desert cats find lairs in rock crevices. For large surface animals, such as the Arabian Oryx, the gazelles and the hares, thermal insulation is a great problem. There is rarely more shade than that under an overhanging rock or bush or a shallow cavity scratched out in the sand.
The most serious problem facing desert animals is that of water balance. Smaller mammals can obtain moisture by feeding on succulent plants and insects. The small shrub Rhanterium eppaposum, or Arfaj in Arabic, is a water supplying shrub for larger animals like the gazelles and camels. Salt bush is also a source of moisture in areas near the coast. Arabian gazelles have been observed to drink sea water, when fresh water is not available.
The came is an unusual animal, well adapted to desert life. It can store up to 30 pounds of fat in its hump and later convert this into energy and water. To conserve water, it can tolerate a 9°F rise in its body temperature. To replenish its body, it can drink up to 25 gallons of high-salt-content, brackish water and its kidneys and body cells tolerate and process the salt.
For plant eating animals, the scarcity of pasture in periods of prolonged drought is another great problem. The larger animals like the Arabian oryx can cover great distances to discover those favored areas where rain has recently fallen. In fact, they do not have to have free water for months at a time, living on moisture from succulent plants or even occasionally dew. The oryx is one of the few animals that can consume the bitter desert gourd (Citrullus).
The smaller mammals are less fortunate and must retire deep into their burrows and live on food stored in better times. Many side tunnels in the burrows are used as storage chambers for seeds collected to see out the long, hot summer days. Some may even aestivate to reduce the need for energy expenditure.
When rain does fall on the desert plateau, some of it seeps down to form reservoirs underground. Plants in wadis or run-off areas usually have long tap roots and are usually the woody types like acacia (as seen along the road to Al Ain). Horizontal roots hold firm. Most other plants of the desert are short-lived. They grow quickly after rain and many contain a water soluble retardant, which will only dissolve after a good soaking. They have an extremely short life cycle and flower and fruit in a few weeks. Many desert plants are succulents with the ability to store water internally. Roots are shallow and some have spines. Salt bush can tolerate salt water, and shrubs such as tamarisk secrete salt from the tips of their needle-like leaves.
Scorpions, sulfugids and beetles are usually covered with a hard shell. In the scorpion, the cuticle is hard and waxy, preventing the evaporation of water. Like small mammals, they tend to remain hidden during the hottest part of the day. They usually obtain all the moisture they require from the food they eat, which could be small lizards or insects.
Snakes and lizards are cold-blooded animals, which means they are unable to regulate or maintain their internal body temperature. Like small mammals they spend much of their time in burrows only coming tout when the temperature is around 86°F (30°C) and are most active around 95°F (35°C). They can usually be found between the times of 8 to 12 in the morning and 3 to 6 in the afternoon during the hot summer days. The food of snakes is usually lizards, small mammals and insects, while lizards eat mainly insects. The desert monitor may be the exception, as it lives on carrion and is known to attack the spiny-tailed agamid, which in turn is vegetarian, eating vegetation and seeds, only occasionally taking locusts.
A few birds remain in the desert and many are known to breed around the outskirts. The ostrich was once resident in this area and it is reported that the last one was shot in 1941. Like the camel, it was able to tolerate salty water. Birds living in the desert usually have to fly to find water as the food they eat provides only part of the water required to replace that lost in body temperature regulation. Most birds keep cool by panting or seeking out shade beneath desert bushes.
(This is based largely on an article by David L. Harrison, author of Footsteps in the Sand and the three-volume standard reference work Mammals of Arabia [Ernest Benn Ltd., London])
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